Centrifugal separators are usually provided with different kinds of braking devices. The braking devices may consist of a mechanical brake acting on the rotor body or on the driving shaft that sets the rotor body in motion. The rotor body may also be retarded in that the motor, which directly or indirectly makes the driving shaft rotate, is switched such that it works as a brake instead. It may take between 5 to 10 minutes to make the rotor body stop completely in a planned shutdown, since the rotating mass of the rotor body is large.
A centrifugal separator with a liquid brake is shown in DE 178 26 47. The liquid brake comprises a ring with a U-formed cross section arranged on a stationary stand. When the centrifugal separator shall be retarded the U-formed ring is filled with liquid. A braking disc, which is arranged on the rotor body, is arranged to stick into the U-formed ring. When the ring contains liquid the friction between the braking disc and the liquid creates a retarding force. In the patent specification there is also described an arrangement where the disc is stationary, while the U-ring is arranged on the rotating drum.
As a security measure centrifugal separators intended for certain applications are provided with vibration alarms, which give a signal if a disturbing unbalance or dangerous vibrations occur. When an alarm has been triggered one tries to retard the body with the braking device. If the unbalance has occurred during a complete or a partial discharge of the collected sludge from the centrifugal separator, one tries to reduce the unbalance by filling the rotor body with water and closing the flow of liquid entering the centrifugal separator. In both these cases it will be a certain time before the rotary speed is so low that the unbalance does not cause any damages.